1. Field of the Invention
The present invention relates to an impregnated flat gasket such as, in particular, a cylinder head gasket for internal-combustion engines, made of soft material fiber mats which may be reinforced with a metal reinforcing member, and to a method of producing such an impregnated flat gasket.
2. Background of the Art
A cylinder head gasket for internal-combustion engines is preferably composed of fiber mat material which is plate-shaped and which may contain asbestos or be asbestos free. A metal carrier sheet may be laminated between a pair of fiber mats for reinforcement. Primarily to improve its strength and its sealing behavior, the fiber mat material is preferably impregnated with liquid polymers that are able to undergo cross-linking. These polymers are then cross-linked in the fiber mat, preferably thermally, and by employing cross-linking agents, such as catalysts, as is known in the art. Passage openings are punched through the fiber mat, for example, for internal combustion engine members including the combustion chamber, the fluid to be cooled, the coolant, and the fastening screws. The cut edges of passage openings that are to be under high thermal stresses in use are protected by encasement within U-shaped metal borders that are bent around the edges.
Cylinder head gaskets have been produced by rolling fiber mats onto both sides of a metal carrier sheet, preferably a metal sheet provided with rough teeth, the rough teeth mechanically fastening and clamping together the fiber mats to provide a laminate. Thereafter, the gasket outline and passage openings of the cylinder head gasket blank are punched out of the resulting laminate. Only after the respective passage openings of the gasket blank have been encased in U-shaped metal borders is the finished gasket impregnated with a cross-linkable impregnating agent and then the impregnating agent is cross-linked. The prior art's technique of impregnating the encased gasket advantageously eliminates the need for punching out already impregnated material, a task characterized by a great amount of punching work and tool wear. At the same time, as described in published German Patent Application No. 2,304,592, the impregnation of the already encased gasket keeps the fiber mat material underneath the metal borders substantially free of impregnating agent so that the material there can be subjected to significantly greater pressure stresses than impregnated fiber mat material is able to withstand. Thus, such prior art gaskets can be subjected to high sealing pressures and produces the desired high sealing effect along the passage opening edges.
On the other hand, impregnation of gaskets that are already punched and provided with edge encasements is expensive and may, under the influence of various factors, result in gaskets with faulty impregnation. The cut outline edges of the gasket and the cut edges of the passage openings which are not protected by edge encasements have a greater suction capability for impregnating agent so that more impregnating agent tends to enter into the fiber mat material in the regions around these cut edges. Thus, density differences result in the impregnated soft material, with increased swelling behavior in the edge zones. The presence of impregnating agent in the fiber mat and its subsequent cross-linking produces swelling of the fiber mat so that the gasket must subsequently be further compressed and the borders must be leveled which is expensive.
These difficulties can be avoided if the sealing material is impregnated before the gaskets are punched out. However, it has been noted that, if prior art sealing materials are employed that are made of impregnated and metal reinforced fiber mats, the above-described drawbacks reoccur during punching and as edges are exposed. Moreover, such gaskets no longer have the required absence of impregnating agent underneath the borders.